The serologic markers of rabbit immunoglobulins-allotypes and idiotypes, are under study because they can provide information concerning the number and arrangement of genes encoding the immunoglobulins. A major effort has been made to characterize allotype b4v, a new allele of the group b allotypes of the CK region. This allotype, first discovered in this laboratory during amino acid sequence studies, has now been shown by genetic analysis to be an allele of the other group b allotypes and has been characterized serologically. The utility of chain specific idiotypes for genetic studies in the rabbit has been confirmed, and the initial study of an L chain specific idiotype has been extended with an H chain-specific idiotype. It was possible to show linkage of this idiotype to VH and CH allotypes within a single allogroup, and to demonstrate latent expression of an allogroup. Attempts to improve breeding efficiency of inbred rabbits are underway at present. Idiotypy studies will be extended to these animals when sufficient numbers are available.